This invention relates to a method and composition for preventing the fouling of submerged objects, and more particularly to a method and composition for preventing the fouling of marine structures which for extended periods of time are left at dock or are in environments where water rapidly flows over the submerged surface.
There are two basic types of anti-fouling paints. One type functions via a leaching mechanism and involves the use or inorganic and/or low molecular weight or monomeric organic active anti-fouling agents which at a controlled rate dissolve through the coating matrix into the sea, thereby functioning as a biocide. However, as the materials leach out through the surface of the coating, the coating matrix which remains exhibits an increasingly rough and uneven texture and appearance. In addition, the speed of ships coated with these materials may actually be significantly reduced, particularly in the case of coatings where a major amount of leaching has occurred. Finally, while multiple coats of traditional leaching anti-fouling coatings may be applied to a ship, the total number of coats is limited by the ability of inner layers to leach through the depleted matrix to the surface. An example of the materials which may be used in these leaching anti-fouling paints is basic copper sulfate, as described in U.S. Pat. No. 3,615,744. This material is used in combination with 2-amino-3-chloro-1,4-naphthoquinone.
The second and more recent development in anti-fouling paints is based upon compositions utilizing an organotin polymeric product. These polymeric systems operate by an ablative mechanism, whereby, the entire exposed surface slowly dissolves into sea water, leaving behind a smooth coating surface. Accordingly, even though many layers of ablative materials are applied, each layer as it becomes exposed to sea water effectively functions as an anti-fouling coating. In addition, because of the smoothness of the coating surface, regardless of the amount of ablation which has occurred, the speed of ships coated with these materials is not adversely affected, but actually may be enhanced. An example of the materials of this latter type is set forth in U.S. Pat. No. 4,174,339, which discloses the use of copper thiocyanate an an anti-fouling pigment in an organotin-based polymeric system.
One of the major deficiencies in prior art ablative anti-fouling paints has been their ineffectiveness in calm or stagnant water environments. In other words, most ablative anti-fouling paint compositions work well when there is a fairly rapid flow of water across the surface to be protected. Such a water flow helps to increase the rate of ablation and simultaneously washes away bacterial slime and silt. However, when a water flow is not present, as with piers, along protected harbors, and especially with ships at anchor, little or no surface washing takes place. Moreover, the longer submerged marine surfaces are kept in still water, the greater the marine growth build-up becomes, and even if a period of surface flushing occurs, it often is not of sufficient duration or strength to remove the large incrustations, barnacles, and other fouling materials. This problem is particularly severe because under current economic conditions, large numbers of ships are left unused or are required to sit at anchor often for extended periods of time. Thus, there is a need for ablative anti-fouling paints which work well in calm waters, and particularly on ships at extended anchor.
Accordingly, it is an object of this invention to provide an anti-fouling paint which protects the hulls of ships and other marine structures from fouling. It is another object of this invention to prepare an ablative anti-fouling paint composition which may be used in calm waters and on ships at anchor for long periods of time. These and other objectives are obtained by preparing the composition of this invention.